How do we span this door threshold cavity.?

[MIJ85]

Our attempts to replace few cracked tiles by the back door has uncovered a can of worms.
The cavity wall had been filled to surface by insulating material overlain by a layer of mortar - now removed leaving us with a hole ! Should we fill it with insulation?
It has been suggested that we span the cavity and create an internal sill with a piece of MDF window sill. We would need us to bolt through the external wall to support it somehow.
Is this going to give us problems with damp - or excessive heat loss?
Any better ideas?

 

[Alastairreid]

Clear it out, insert a strip of Dpm to form a pocket and backfill level to finished floor level.

 

[MIJ85]

Thanks for that .. to do mean 5 sided pocket ? ie - base and 4 sides?
Sould we fill with insulation plus cement - or just cement? - How thick?
Sorry for the ignorance!

 

[Alastairreid]

Yes you are creating a tray to stop any moisture from getting through from the external wall or ground..100mm fill will be adequate, insulation if you have any lying around won't do any harm.

 

[Tigercubrider]

A strong poly bag opened up would give you enough material to fold up to make a pocket to backfill. Fold not cut so it’s waterproof.
Remove rubble and add tamped down sand so nothing pokes through the poly

you can make a simple level gauge by screwing a wooden block to a plank, the bottom of the plank sits on the final floor surface and the block represents whatever level the concrete needs to be.

don’t use MDF. Use a piece of hardwood.

 

[noseall]

Our attempts to replace few cracked tiles by the back door has uncovered a can of worms.
The cavity wall had been filled to surface by insulating material overlain by a layer of mortar - now removed leaving us with a hole ! Should we fill it with insulation?
It has been suggested that we span the cavity and create an internal sill with a piece of MDF window sill. We would need us to bolt through the external wall to support it somehow.
Is this going to give us problems with damp - or excessive heat loss?
Any better ideas?
View attachment 259912 View attachment 259914

For a start you need to treat it as any other door opening and remove at least a course (maybe two) of those internal bricks.
Put sand in the bottom to line it smooth.
Add a DPM trough fully lining the all exposed masonry.
Add insulation (trough bottom and outside wall).
Fill with conc.